A known method for the drilling cavities or shafts in ground layers consisting of rock, clay and/or related materials comprises of arranging a borehole casing in the ground, lowering into the borehole casing a hollow drill string provided with a drill head with cutting tools, then setting the drill string into rotation in the borehole casing so that ground material is dislodged by the cutting action of the cutting tools, and discharging the dislodged ground material, for instance by suctioning through the cavity of the drill string.
The known method has the drawback, among others, that during drilling in cohesive ground layers, such as for instance in clay, ground material remains adhered to the drill head, whereby its cutting action is impeded. Not only is less ground material dislodged, the discharge of the dislodged ground material is moreover obstructed. Both effects result in a reduced drilling efficiency. Similar problems otherwise occur when drilling in cracked rock and in compact sand layers.